A MOPA is known in the art as a specific type of optical amplifier that comprises at least two separate elements, a laser source (the “master oscillator”) and an optical amplifier. At times, the laser source is referred to as the “seed laser”. As this name implies, the laser source is used to “seed” an optical amplifier with an input trigger signal that then generates a high power output signal pulse. By virtue of using a separate power amplification component, the various performance aspects of the laser source itself are decoupled from the requirements of the power generator. Indeed, the MOPA technology provides an efficient power scaling architecture for pulsed laser applications such as “light detection and ranging” (LIDAR). LIDAR is a technology that can be used to measure distances to remote targets, with a laser source used to generate optical pulses that are amplified and directed toward a target which then scatters the light. The separate power amplifier within the MOPA can thus be independently controlled to provide the desired amount of signal gain for a given LIDAR application. Some of the scattered light is received at a detector co-located with the laser source and the distance to the target is then determined based on one or more characteristics of the returned light.
In many LIDAR applications, a relatively high power (e.g., on the order of hundreds of watts) optical pulse is required so that the scattered, returned light has enough power to yield accurate distance calculations. For some applications (such as, for example, on-board automotive LIDAR systems), the MOPA is configured to generate extremely narrow output pulses and thus requires the use of nsec-scale seed laser pulses. The amplifier portion typically comprises a fiber-based (rare-earth) amplifier (such as an erbium-doped fiber amplifier, EDFA) that utilizes pump light at an appropriate wavelength (e.g., 980 nm) to excite the rare-earth ions in the fiber and thereby amplify the seed laser input signal pulses to a power level sufficient for the required “high power” output pulses. The seed laser is controlled to exhibit a predetermined pulse repetition rate. Instead of defining the input signal pulse train in terms of repetition rate, it is also common to define the pulse train by its “pulse repetition interval” (PRI), which defines the time interval between adjacent pulses (typically measured from the rising edge of a first pulse to the rising edge of a second pulse).
For applications such as LIDAR, there is a need to vary the PRI over an extended period of time to account for constant changes in the surrounding area being surveyed. Changing the PRI has been found to create a transient change in output energy, which is attributed to changes in the amount of amplified spontaneous emission (ASE), radiative noise, produced as the PRI is changed. Previously, this problem has been addressed by controlling the drive current applied to the pump source so as to modify the amount of pump energy available as a function of changes in PRI. This not considered as a satisfactory solution in many applications, such as MOPAs utilizing multiple PRIs that change rapidly as a function of time, since the gain response of the amplifier may not be managed as quickly as the change in PRI.